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G. B. Hansen, T. B. McCord (Hawaii Inst. of Geophys. & Planet., Univ. Hawaii)
The lattice order of ice depends on its condensation rate, temperature, and exposure to high energy particles or photons. The surface temperatures of the icy Galilean satellites of Jupiter are such that ice will crystallize over very short time scales. But amorphous ice is easily created by the disruption of crystalline ice by radiation, which is strong throughout the Galilean system. We envision a balance between thermal crystallization and radiolytic disruption, where low temperatures and high radiation favor amorphous ice on Europa, while opposite conditions favor crystalline ice on Callisto. Ganymede is an intermediate case whose intrinsic magnetic field diverts much of the local plasma to high latitudes, suggesting that the bright polar caps may be amorphous. We use 0.7-5.3 \mum spectra returned by the Near-Infrared Mapping Spectrometer on the Galileo spacecraft to study the crystallinity of the surface ice on all three satellites. We analyze the Fresnel reflection peak from grain surfaces near 3.1 \mum, which is stronger and has a more detailed spectrum for crystalline ice. We identify amorphous ice by its broad and featureless peak, and crystalline ice by its narrower three-peaked structure. Despite conditions which limit the study of Europa (high noise) and Callisto (little ice) to regional averages, we have verified that the surface ice is mostly amorphous on Europa, and crystalline on Callisto. On Ganymede, there is more ice and lower noise, so we are mapping the distribution of crystalline and amorphous surface ice using the properties of the 3.1-\mum peak. Preliminary maps show that this distribution is very detailed and not a simple latitudinal gradient due to radiation. The general trend shows more crystalline ice at low latitudes and more amorphous ice at the poles, but local conditions apparently dictate large variations in this trend, perhaps related to the surface albedo or radiation environment.
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